The present invention relates to a magnetic random access memory (MRAM) device, and more particularly, to a spin transfer torque (STT) MRAM device including therein an ultrathin perpendicular reference layer.
Spin transfer torque magnetic random access memory (STT-MRAM) is a new class of non-volatile memory, which can retain the stored information when powered off. An STT-MRAM device normally comprises an array of memory cells, each of which includes at least a magnetic memory element and a selection element coupled in series between appropriate electrodes. Upon application of an appropriate voltage or current to the magnetic memory element, the electrical resistance of the magnetic memory element would change accordingly, thereby switching the stored logic in the respective memory cell.
FIG. 1 shows a conventional memory element for an STT-MRAM device comprising a magnetic reference layer 50 and a magnetic free layer 52 with an insulating tunnel junction layer 54 interposed therebetween, thereby collectively forming a magnetic tunneling junction (MTJ) 56. The magnetic reference layer 50 and free layer 52 have a fixed magnetization direction 58 and a variable magnetization direction 60, respectively, which are substantially perpendicular to the layer planes thereof. Therefore, the MTJ 56 is a perpendicular type comprising the magnetic layers 50 and 52 with perpendicular anisotropy. Upon application of an appropriate current through the perpendicular MTJ 56, the magnetization direction 60 of the magnetic free layer 52 can be switched between two directions: parallel and anti-parallel with respect to the magnetization direction 58 of the magnetic reference layer 50. The insulating tunnel junction layer 54 is normally made of a dielectric material with a thickness ranging from a few to a few tens of angstroms. However, when the magnetization directions 60 and 58 of the magnetic free layer 52 and reference layer 50 are substantially parallel, electrons polarized by the magnetic reference layer 50 can tunnel through the insulating tunnel junction layer 54, thereby decreasing the electrical resistivity of the perpendicular MTJ 56. Conversely, the electrical resistivity of the perpendicular MTJ 56 is high when the magnetization directions 58 and 60 of the magnetic reference layer 50 and free layer 52 are substantially anti-parallel. Accordingly, the stored logic in the magnetic memory element can be switched by changing the magnetization direction 60 of the magnetic free layer 52.
The magnetic reference layer 50 often includes therein a thick multilayer or superlattice structure for enhancing the perpendicular anisotropy. A typical multilayer structure has a thickness range of several to tens of nanometer and thus a relatively large magnetic moment. The large magnetic moment of the multilayer structure and a strong dipole coupling between the magnetic reference layer 50 and free layer 52 produce a large stray field exerted on the magnetic free layer 52, which reduces the stability of the anti-parallel magnetization configuration and makes the parallel-to-anti-parallel switching current higher than anti-parallel-to-parallel switching current. The stray field exerted on the magnetic free layer 52 by the magnetic reference layer 50 may be partially or completely canceled by adding magnetic layers with a fixed magnetization direction that is opposite to the fixed magnetization direction 58 of the magnetic reference layer 50. However, doing so would undesirably increase the total thickness of the magnetic layers in the memory element, thereby adversely reducing the etching process margin for thick MTJ layer stack in a dense array.
For the foregoing reasons, there is a need for an STT-MRAM device having a minimum stray field at the magnetic free layer while the total thickness of the MTJ memory element is minimized.